In a fast reactor plant, elevated-temperature liquid metal sodium which has been used to cool the inside of a nuclear reactor is introduced to a steam generator where heat is exchanged with that of water to generate steam. In this case, a tube material of double-wall structure (double-wall tube) in which an outer-wall tube and an inner-wall tube are mechanically brought into close contact with each other is used as a heat-transfer tube constituting the above-mentioned steam generator. It is for the following two reasons that the double-wall tube is used as the heat-transfer tube constituting the steam generator.
One reason is that the double-wall tube is excellent in cracking resistance. Within the steam generator, water is passed through the inside of the heat-transfer tube, while liquid metal sodium travels around the outside thereof. At that time, if a crack penetrated through in thickness direction occurs in the heat-transfer tube, the liquid metal sodium should contact with the water to cause an extremely dangerous explosive reaction.
In a solid tube material of a single wall structure, a surface defect generated on either the inner surface or outer surface thereof is apt to propagate to the other surface, causing a crack penetrated through in a thickness direction. On the other hand, in the double-wall tube in which the inner-wall tube and the outer-wall tube are only mechanically joined to each other, there is no risk that a crack generated on a wall surface is immediately propagated to the other wall surface to form a crack penetrated through both the thicknesses of the inner- and outer-wall tubes. Therefore, the double-wall tube excellent in cracking resistance is used as the heat-transfer tube constituting the steam generator.
The other reason is that the failure of the double-wall tube can be detected at an early stage. In the use of the double-wall tube as the heat-transfer tube constituting the steam generator, if a crack occurs in either the inner-wall tube or the outer-wall tube, a leaked fluid due to the crack is oozed to a tube end through a small gap between the outer-wall tube and the inner-wall tube. This leaked fluid to the tube end is detected, whereby the failure of the double-wall tube can be detected at an early stage.
However, in a double-wall tube simply composed of a smooth-surface outer-wall tube and a likewise inner-wall tube in which a gap between the two tubes is as narrow as a few microns, the detection of the crack is delayed since it takes long time until the leaked fluid is oozed to the tube end after the occurrence of the crack. On the other hand, the use of the double-wall tube as the heat-transfer tube requires excellent heat conductivity without a gap between the outer-wall tube and the inner-wall tube.
Therefore, a large number of proposals are made with respect to a double-wall tube configured to secure a flow passage for a leaked fluid between an inner-wall tube and an outer-wall tube by interposing a porous layer between the two tubes and to enhance the degree of contacting of the porous layer or secure excellent heat conductivity by sufficiently bringing the porous layer into close contact with the inner surface of the outer-wall tube and the outer surface of the inner-wall tube, and a method for producing the same.
For example, a method for producing a double-wall tube for a fast-breeder reactor is proposed in Patent Literature 1, wherein a porous metal layer interposed between an inner-wall tube and an outer-wall tube is surely brought into close contact with one or both of the inner-wall and outer-wall tubes by heat-treating a double-wall tube obtained by interposing an insert material between the mating surfaces of the porous metal layer and the inner-wall and outer-wall blank tubes, followed by diameter reduction after air-tightly sealing the mutually mating surfaces of the inner-wall and outer-wall tubes at both ends of the tube.
In Patent Literature 2, a double-wall heat-transfer tube for a steam generator is disclosed, in which the filling rate of a porous metal to a gap portion between an outer-wall and an inner-wall blank tubes is set in a range of 70% to 95%, a surface roughness of at least either the inner surface of the outer-wall blank tube or the outer surface of the inner-wall blank tube before double-wall-tube processing is set in a range of 0.5 μm to 1.6 μm and, further, the porous metal is constituted by braiding a plurality of thin wires differed in wire diameter. For obtaining this double-wall heat-transfer tube, a method for producing a double-wall heat-transfer tube is also disclosed therein, in which an outer-wall blank tube and an inner-wall blank tube are solid-phase diffusion bonded to a porous metal respectively by performing heat treatment while holding a gap portion, surrounded by the outer-wall tube, the inner-wall tube with an interposed porous metal, under vacuum.
In Patent Literature 3, a heat-transfer tube for a steam generator which is excellent in both crack detecting performance and heat transfer performance is disclosed, in which the heat-transfer tube includes an inner-wall tube and an outer-wall tube composed of iron-based alloy, and a porous body excellent in heat conductivity and having a porosity of 3%≧, which is interposed between the two tubes, and the porous body is bonded to the inner-wall tube and the outer-wall tube through metal coating layers formed respectively on the outer surface of the inner-wall tube and the inner surface of the outer-wall tube. As a method for producing this heat transfer tube, a method for producing a heat-transfer tube for a steam generator is also disclosed therein, in which the porosity of a porous body to be inserted between an inner-wall blank tube and an outer-wall blank tube is set to 30% or more, the reduction rate of drawing is set to 70% or less, or metal coating layers are preliminarily formed respectively on the outer circumferential surface of the inner-wall blank tube and the inner circumferential surface of the outer-wall blank tube by means of electroplating or the like.
In Patent Literature 4, a method for producing a double-wall heat-transfer tube is disclosed, the method comprising the steps of; inserting, into an outer-wall blank tube, an inner-wall blank tube with a ceramic coating layer formed on the outer surface, the inner-wall blank tube being enhanced in heat conductivity by reducing the thickness of the coating layer; and generating, in the ceramic coating layer, a crack that forms a leak detection flow path for fluid to be heated while plastically deforming the inner-wall blank tube by a tube expanding process.
However, the producing of each of these conventional double-wall tubes with a porous body interposed between an inner-wall tube and an outer-wall tube requires respective specific producing processes for maintaining the porosity for securing the flow path for a leaked fluid and for maintaining satisfactory heat conductivity.
Namely, the method described in Patent Literature 1 requires the steps of interposing the insert material between the mating surfaces of the porous metal layer and each of the inner-wall and outer-wall blank tubes and performing diameter reduction by use of a plug. The method described in Patent Literature 2 requires the specification of the filling rate of the porous metal to the gap portion between the outer-wall and the inner-wall blank tubes and the surface roughness of the inner surface of the outer-wall blank tube and the outer surface of the inner-wall blank tube prior to a double-wall tube processing, and further the treatment in vacuum for the solid-phase diffusion bonding. The method described in Patent Literature 3 requires the specification of the porosity of the porous body and the reduction rate of drawing, or the preliminary formation of the metal coating layers on the outer circumferential surface of the inner-wall blank tube and the inner circumferential surface of the outer-wall blank tube by electroplating or the like. The method described in Patent Literature 4 requires processes for the formation of the ceramic coating layer on the inner-wall blank tube surface by PVD or CVD, and the formation of the crack in the coating layer by the tube expanding process of the inner-wall blank tube.